This invention relates to the use of tangential firing systems in fuel-fired steam generating power plants and more specifically to a sixpoint separated overfire air tangential firing system employed in power plants which utilize dual-chambered furnaces.
In general the tangential firing technique involves the introduction of combustible fuel and air into a furnace volume from multiple locations about the perimeter of the furnace volume in such a manner that the fuel and air are directed tangent to an imaginary circle which lies in a horizontal plane and is concentric with the furnace volume. The fireball that results from the combustion of the fuel and air thus rotates about the center of the imaginary circle. This type of firing has many advantages, among them being good mixing of the fuel and air, stable flame conditions and long residence time of the combustion gases in the furnace.
In recent years, due to very strict state and federal environmental regulations demanding that emissions be maintained at acceptable levels, greater emphasis has been placed upon minimizing air pollution generated from these power plants. To this extent, oxides of nitrogen, also known as NO.sub.X, have beet implicated as one of the constituents that contribute to the generation of acid rain and smog.
Oxides of nitrogen are a byproduct of the combustion of hydrocarbon fuels, such as pulverized coal in air, and are found in two main forms. If the nitrogen originates from the air in which the combustion process occurs, the NO.sub.X is referred to as `thermal NO.sub.X.` Thermal NO.sub.X forms when very stable molecular nitrogen, N.sub.2, is subjected to temperatures above about 2800.degree. F. causing it to break down into elemental nitrogen, N, which can then combine with elemental or molecular oxygen to form NO or NO.sub.2. The rate of formation of thermal NO.sub.X downstream of the flame front is extremely sensitive to local flame temperature and somewhat less so to the local mole concentration of oxygen. Thermal NO.sub.X concentration can be reduced by lowering the mole concentrations of N.sub.2 and O.sub.2, reducing the peak flame temperature and reducing the amount of time that N.sub.2 is subjected to these temperatures.
If the nitrogen originates as organically bound nitrogen within the fuel, the NO.sub.X is referred to as `fuel NO.sub.X.` The nitrogen content of coal, for instance, is comparatively small and, although only a fraction is ultimately converted to NO.sub.X, is the primary source of the total NO.sub.X emissions from a steam generating power plant. The formation rate of fuel NO.sub.X is strongly affected by the rate of mixing between the fuel and air stream in general, and by the local oxygen concentration in particular. The formation of fuel NO.sub.X is a multi-stage process. During initial coal particle heat up the coal is broken down into both volatile matter consisting of reactive cyanogens, oxycyanogens and amine species and char consisting of unburned carbon, hydrocarbons and ash. In an oxygen rich environment the volatile matter will convert largely to NO.sub.X and in a fuel-rich environment it can be reduced to N.sub.2. The remaining fuel-bound nitrogen is released during combustion of the carbon based byproducts, i.e. char, of the combustion of the coal particles. For char combustion to approach completion, an oxygen rich process is required. As with the volatile released NO.sub.X, the eventual fate of char released nitrogen is dependent upon the specific time, temperature and stoichiometric history.
The stoichiometric ratio, .phi., of a combustion process is defined here as the number of moles of oxygen supplied to combust a given quantity of fuel divided by the number of moles of oxygen theoretically necessary to combust the same quantity of fuel. Typically, the stoichiometric ratio in a fossil fuel-fired steam generating power plant is a quantity greater than or equal to one and can be expressed as a percentage in which case it is referred to as percent theoretical air, .tau.=.phi..times.100. A related term is excess air which is (.phi.-1).times.100 or .tau.-100.
From the preceding it should be apparent that by controlling the distribution and mass flow rate of air to the combustion process the stoichiometric ratio of the process can be controlled and thus also the formation of NO.sub.X. One method of controlling the distribution and mass flow rate of air to the combustion process occurring within a tangentially fired furnace is through the use of staged combustion. In accordance with staged combustion typically there is defined a main burner zone within the furnace volume. Within the main burner zone fuel, is initially only partially combusted in a fuel-rich environment by withholding a portion of the total air necessary for complete combustion. Next that portion of air which has been withheld from the main burner zone, and which is sometimes referred to as overfire air (OFA), is introduced into the furnace volume above the main burner zone, frequently at multiple elevations. A typical configuration of overfire air is found in the utilization of one or two closely grouped compartments at a single, fixed elevation near the top of a plurality of vertically arrayed compartments housed within a common vertical plenum known as a windbox. This overfire air is generally referred to as close coupled overfire air (CCOFA). In addition, at a higher elevation, and separated from the main windbox, separated overfire air (SOFA) is introduced into the furnace volume from a similar group of compartments located in a SOFA windbox. The use of separated overfire air in a tangentially fired furnace can be seen, by way of exemplification and not limitation, in U.S. Pat. Nos. 5,020,454, 5,195,450, 5,315,939 and 5,343,820, each of which teaches the use of a plurality of separated overfire air compartments.
Upon introduction into the furnace volume above the main burner zone the separated overfire air is mixed with and finally combusted with the products generated by the incomplete combustion occurring within the main burner zone. The use of close coupled and separated overfire air minimizes NO.sub.X formations via two mechanisms. First, by having a fuel-rich atmosphere in the main burner zone, i.e., a so called substoichiometric condition, the initial amount of fuel NO.sub.X formed is reduced because less oxygen is available to combine with the fuel-bound nitrogen. Second, lower fuel NO.sub.X results because of the reduced air concentrations during the initial firing stage. Thus, this has the effect of increasing the residence time within the main burner zone. To this end, residence time is the amount of time necessary for a fuel particle to combust. The increased residence time provides an environment, which is conducive to the reduction of any oxidizable N.sub.2 volatiles that have been formed such as NH.sub.3 or HCN. This is because the NO.sub.X compounds and the volatiles are entrained and reduced to their elemental components, oxygen and nitrogen, and because the hydrocarbons are combusted. Furthermore, the use of overfire air has the effect of reducing the peak flame temperatures, thereby resulting in lower thermal NO.sub.X formation.
In accordance with a typical configuration of a tangentially fired furnace wherein both staged combustion and overfire air are being utilized, fuel plus primary air, as well as secondary air and close coupled overfire air, are supplied, by means of a forced draft fan and various ducts, to a main windbox. From the main windbox, in accordance with conventional practice, the fuel and air are then introduced into the furnace volume. Primary air is used to transport solid fuel to the windbox and secondary air and close coupled overfire air are used to control the stoichiometric ratio within the main burner zone. The flow rates of secondary air and close coupled overfire air are controlled by individual dampers located within the main windbox. Furthermore, in accordance with a typical configuration of a tangentially fired furnace, main windboxes may be located at or near the corners of the furnace volume with fuel and air directed tangential to an imaginary circle, which lies in a horizontal plane and is concentric with the furnace volume. However, tangential firing may also be accomplished with windboxes located at or near the mid-wall of the furnace sides. U.S. Pat. No. 5,429,060 discloses the use of burners disposed at the central portions of respective sides in a horizontal cross-section of a furnace wall and U.S. Pat. No. 5,315,939 discloses an integrated low NO.sub.X tangential firing system.
In many existing power plants a dual-chambered furnace volume is provided wherein the combustion process takes place. Typically tangential firing is accomplished in these power plants via four main windboxes per chamber suitably located at appropriate points along the front and rear waterwalls, which along with the side waterwalls serve to define the furnace volume. However, such furnaces may not be equipped with close coupled overfire air and/or separated overfire air. Instead, the main windboxes and associated ductwork may simply direct fuel along with primary air and secondary air into the respective chambers. In a dual-chambered furnace volume, the resulting combustion of the fuel and air injected thereto yields two rotating fireballs. Each of these two rotating fireballs is coaxial with the center of a corresponding one of the dual chambers of the furnace volume.
It should be readily apparent from the foregoing that in a tangentially fired furnace, it is desirable to reduce the formation of NO.sub.X during the combustion process itself. This is in fact a matter of critical importance and a major concern in the design and operation of new power plants as well as in the retrofitting of existing plants. Reducing the formation of NO.sub.X during the combustion process obviates the need for expensive post combustion air pollution devices. However, without overfire air and the attendant NO.sub.X reducing capabilities that it affords, it is difficult for power plants not so equipped to meet state and federal NO.sub.X emission requirements. Therefore, in order to meet these requirements, it is desirable to equip the furnaces of these power plants with separated overfire air (SOFA).
In a retrofit application there are, however, numerous difficulties that need to be overcome, before the furnaces of these power plants can, without sacrificing the aforementioned advantages of tangential firing (good mixing of fuel and air, stable flame conditions and long residence time of combustion gases in the furnace), be equipped with separated overfire air. First, it is necessary to maintain the total mass flow rate of air, which is to be introduced into the respective chambers of the multi-chambered furnace volume. This means that the totality of primary air, secondary air, and close coupled overfire air if present, before retrofit, is equal to the totality of primary air, secondary air, close coupled overfire air and the retrofitted separated overfire air. Furthermore, the totality of the mass flow rate of fuel introduced into the furnace volume must remain the same before and after the retrofitting of the separated overfire air. Maintaining the totality of the mass flow rates of air and fuel furthermore must be accomplished while the total mass flow rate of output steam produced by the power plant is also being maintained. In addition, it is also necessary that the management of the fireball remains consistent before and after the retrofitting. By the management of the fireball is meant the ability to maintain the stability of the fireball as well as its shape and location within the respective chambers of the multi-chambered furnace volume. Still further it is necessary to minimize the number of expensive penetrations that are required to be made through the waterwalls of the furnace volume in order to retrofit the separated overfire air. Moreover, it is desirable to do so because of the limited space that is available to accommodate added ductwork. In view of the need to minimize the number of waterwall penetrations, it is also desirable to accomplish the retrofitting of the separated overfire air with a minimum number of SOFA windboxes.
The present invention is capable of addressing these needs while maintaining the low NO.sub.X advantages of tangentially fired staged combustion. This is achieved by providing six SOFA windboxes strategically located about the perimeter of each chamber of a dual-chambered furnace volume at an elevation above the main burner zone and oriented so as to inject separated overfire air into each chamber in such a manner that the mixing of the separated overfire air with the flue gases generated therein is substantially equivalent whether three SOFA windboxes are used or four SOFA windboxes are used.
As alluded to above, it is essential for purposes of achieving efficient combustion that there be as thorough and complete a mixing of fuel and air as possible. To that end, it has been found that, in accordance with the present invention, separated overfire air delivered to the respective chambers of a dual-chambered furnace volume by means of three strategically spaced SOFA windboxes per chamber results in a mixing of separated overfire air and the combustion gases in the furnace volume that is virtually identical to the mixing which is achieved through the utilization of more than three SOFA windboxes per chamber, e.g., four SOFA windboxes per chamber. Furthermore, in the present invention, flame stability, shape and location within the chamber are not degraded when three SOFA windboxes are used as opposed to the use of more than three SOFA windboxes, e.g., four SOFA windboxes. Also in the present invention, a furnace volume retrofitted with three SOFA windboxes will still enable that there be the long residence time of the combustion gases in the furnace volume which is inherent with the employment of the overfire air concept to be realized. Also in accordance with the present invention it is still possible to maintain the totality of the mass flow rates of fuel and air delivered to the furnace volume while yet maintaining the totality of the mass flow rate of output steam of the power plant when the subject matter of the present invention is employed in a retrofit application. Furthermore, in accordance with the present invention a minimum number of expensive waterwall penetrations and attendant ductwork associated with SOFA windboxes are required. Still further, the subject matter of the present invention, although primarily intended for application in dual-chambered furnace volumes, is equally applicable to both dual-chambered and single-chambered furnace volumes.
It is, therefore, an object of the present invention to provide a new and improved separated overfire air system which is particularly suited for use in a dual-chambered furnace volume.
It is also an object of the present invention to provide such a new and improved separated overfire air system for use in a dual-chambered furnace volume, which is characterized in that through the use thereof the mass flow rate of fuel and the mass flow rate of air delivered to the respective chambers of the dual-chambered furnace volume remains unchanged after being retrofitted therein.
It is a further object of the present invention to provide such a new and improved separated overfire air system for use in a dual-chambered furnace volume, which is characterized in that through the use thereof the mass flow rate of output steam remains unchanged after being retrofitted therein.
It is still a further object of the present invention to provide such a new and improved separated overfire air system for use in a dual-chambered furnace volume, which is characterized in that through the use thereof the ability to maintain fireball stability, shape and position within the respective chambers of the dual-chambered furnace volume is unaffected when retrofitted therein.
It is yet a further object of the present invention to provide such a new and improved separated overfire air system for use in a dual-chambered furnace volume which is characterized in that the use thereof necessitates only a minimum number of waterwall penetrations and attendant ductwork for purposes of effecting the retrofit thereof therein.
It is also an object of the present invention to provide such a new and improved separated overfire air system for use in a dual-chambered furnace volume, which is characterized in that through the use thereof it is possible therewith to effect a reduction in the formation of NO.sub.X in the combustion process occurring within the respective chambers of the dual-chambered furnace volume.
It is still further an object of the present invention to provide such a new and improved separated overfire air system for use in a dual-chambered furnace volume which is characterized in that through the use thereof it is possible to achieve mixing of separated overfire air with combustion gases in the furnace volume which is very nearly equivalent from the use of three SOFA windboxes to that attainable with more than three SOFA windboxes, e.g., four SOFA windboxes.
It is another object of the present invention to provide such a new and improved separated overfire air system for use in a dual-chambered furnace volume which is characterized in that it is equally well suited for use either in retrofit applications or in new applications.
It is yet another object of the present invention to provide such a new and improved separated overfire air system, which is characterized in that it is employable in both dual-chambered as well as single-chambered furnace volumes.
It is still further an object of the present invention to provide such a new and improved separated overfire air system for use in a dual-chambered furnace volume, which is characterized in that it is relatively easy to install, relatively simple to operate, yet is relatively inexpensive to provide.